Nucleic-Acid Transfection
Nucleic acids can be delivered to cells both in vitro and in vivo by pre-complexing the nucleic acids with charged lipids, lipidoids, peptides, polymers or mixtures thereof. Such transfection reagents are commercially available, and are widely used for delivering nucleic acids to cells in culture. Cells exposed to transfection reagent-nucleic acid complexes may internalize these complexes by endocytosis or other means. Once inside a cell, the nucleic acid can carry out its intended biological function. In the case of protein-encoding RNA, for example, the RNA can be translated into protein by the ribosomes of the cell.
Many variables can affect the efficiency of reagent-based transfection, including the structure of the transfection reagent, the concentration of the nucleic acid, and the complex-formation time. Designing a transfection protocol is made even more difficult by the fact that adjusting these variables to increase transfection efficiency often increases transfection-associated toxicity. In addition, several common components of cell-culture media, including serum, some antibiotics, and polyanions such as dextran sulfate or heparin, can inhibit transfection and/or cause cell death when cells are transfected in media containing these components. Thus, the composition of the transfection medium is a critical factor in determining both transfection efficiency and transfection-associated toxicity.
Serum-Free Cell Culture
Animal sera such as fetal bovine serum (FBS) are commonly used as a supplement in cell-culture media to promote the growth of many types of cells. However, the undefined nature of serum makes cells that are contacted with this component undesirable for both research and therapeutic applications. As a result, serum-free cell-culture media have been developed to eliminate the batch-to-batch variability and the risk of contamination with toxic and/or pathogenic substances that are associated with serum.
The most abundant protein in serum is serum albumin. Serum albumin binds to a wide variety of molecules both in vitro and in vivo, including hormones, fatty acids, calcium and metal ions, and small-molecule drugs, and transports these molecules to cells, both in vitro and in vivo. Serum albumin (most often either bovine serum albumin (BSA) or human serum albumin (HSA)) is a common ingredient in serum-free cell-culture media, where it is typically used at a concentration of 1-10 g/L. Serum albumin is traditionally prepared from blood plasma by ethanol fractionation (the “Cohn” process). The fraction containing serum albumin (“Cohn Fraction V” or simply “Fraction V”) is isolated, and is typically used without further treatment. Thus, standard preparations of serum albumin comprise a protein part (the serum albumin polypeptide) and an associated-molecule part (including salts, fatty acids, etc. that are bound to the serum albumin polypeptide). The composition of the associated-molecule component of serum albumin is, in general, complex and unknown.
Serum albumin can be treated for use in certain specialized applications1-3 (US Patent Appl. Pub. No. US 2010/0168000 A1). These treatment processes are most commonly used to remove globulins and contaminating viruses from solutions of serum albumin, and often include stabilization of the serum albumin polypeptide by addition of the short-chain fatty acid, octanoic acid, followed by heat-inactivation/precipitation of the contaminants. For highly specialized stem-cell-culture applications, using an ion-exchange resin to remove excess salt from solutions of BSA has been shown to increase cell viability3. However, recombinant serum albumin does not benefit from such treatment, even in the same sensitive stem-cell-culture applications3, demonstrating that the effect of deionization in these applications is to remove excess salt from the albumin solution, and not to alter the associated-molecule component of the albumin. In addition, the effect of such treatment on other cell types such as human fibroblasts, and the effect of such treatment on transfection efficiency and transfection-associated toxicity have not been previously explored. Furthermore, albumin-associated lipids have been shown to be critical for human pluripotent stem-cell culture, and removing these from albumin has been shown to result in spontaneous differentiation of human pluripotent stem cells, even when lipids are added separately to the cell-culture medium4. Thus, a cell-culture medium containing albumin with an unmodified associated-molecule component is thought to be critical for the culture of human pluripotent stem cells. Importantly, the relationship between the associated-molecule component of lipid carriers such as albumin and transfection efficiency and transfection-associated toxicity has not been previously explored.
Cell Reprogramming
Cells can be reprogrammed by exposing them to specific extracellular cues and/or by ectopic expression of specific proteins, microRNAs, etc.5-9 While several reprogramming methods have been previously described, most that rely on ectopic expression require the introduction of exogenous DNA, which carries mutation risks. These risks make DNA-based reprogramming methods undesirable for therapeutic applications. DNA-free reprogramming methods based on direct delivery of reprogramming proteins have been reported10, 11, however these techniques are too inefficient and unreliable for commercial use. In addition, RNA-based reprogramming methods have been described12-15, however, all previously disclosed RNA-based reprogramming methods are slow, unreliable, and inefficient when applied to adult cells, require many transfections (resulting in significant expense and opportunity for error), can reprogram only a limited number of cell types, can reprogram cells to only a limited number of cell types, require the use of immunosuppressants, and require the use of multiple human-derived components, including blood-derived HSA and human fibroblast feeders. The many drawbacks of previously disclosed RNA-based reprogramming methods make them undesirable for both research and therapeutic use.
Cell-Based Therapeutics
Many diseases are caused by the loss of or damage to one or more tissue-specific cells. Methods for treating such diseases by replacing the lost or damaged cells with cells taken from animals or from one or more human donors have been described. However, the critical shortage of donor cells represents a barrier to the development of cell-based therapeutics for most diseases. In addition, therapeutics based on the use of cells from non-isogenic donors or animals carry a risk of rejection. As a result, patients receiving such cells must take strong immunosuppressant drugs, which themselves carry serious side-effects.